As widely known as for example CD-R (Compact Disc-Recordable) and DVD-R (Digital Versatile Disc-Recordable), write-once type recording mediums having a recording layer whose recording material is an organic dye have been widespread.
Various types of recording mediums using an inorganic recording material have been proposed (for example, Japanese Patent Application Laid-Open No. HEI 11-144316). However, to date, recording mediums using an organic material as a recording material (hereinafter, these recording mediums are referred to as organic recording mediums) have more widespread than recording mediums using an inorganic material as a recording material) (hereinafter, these recording mediums are referred to as inorganic recording mediums). This is because when an inorganic material is used as a recording film of a recording medium, since the flexibility of reflectance ratio is low, the resultant recording medium is not able to have compatibility with a ROM (Read Only Memory). In addition, when the recording characteristics and durability of a recording medium are tried to be improved, a multi-layer film needs to be used, resulting in increasing the facility cost for fabrication equipment for example a spattering unit. Thus, the fabrication cost for recording mediums using an inorganic material becomes higher than that using an organic material.
In contrast, in write-once type optical recording mediums using an organic material as a recording material, since their recording layer can be formed by the spin coat method and a reflection film is formed on the recording layer, the fabrication method is simple and the facility cost of the fabrication equipment is low.
On the other hand, a high density structure of optical discs have been accomplished mainly by using a short wavelength light source and a small numerical aperture (N. A.) objective lens. To date, since blue semiconductor lasers having short wavelengths of around 400 nm have been practically used, there are needs of developing organic dyes suitable for light sources having such wavelengths.
However, in organic dyes that satisfy optical characteristics of light having short wavelengths, the sizes of color dye particles tend to become small. Thus, since the flexibility of molecule design becomes low, the flexibility of designing characteristics of resultant mediums becomes low.
In Blu-ray Disc ((registered trademark), hereinafter referred to as BD) that has been commercialized as a next generation optical disc standard, since its N. A. is 0.85, because the skew tolerance is small, recording and reproducing blue laser light is radiated from a light transmission layer having a thickness of 0.1 mm and formed on the recording layer. When an organic dye is used as a recording material, a dielectric film needs to be formed to prevent molecules of the organic pigment from dispersing in the light transmission layer. In these situations, there is a need for a low cost recording film substituted for an organic dye.
For optical systems for CD-R and DVD-R, an inorganic recording medium may be superior to an organic recording medium in recording and reproducing characteristics. However, in an optical recording medium whose recording layer is formed by the spatter method and that has many structural layers as in a phase change type recording medium, its fabrication process will become complicated and the cost will rise. Thus, it is desired that the number of structural layers of a write-once type recording medium be at most 3 to 4.